1. Field of the Invention
This invention relates to test fixtures for applying loads to a rotary mechanical system and more specifically to dynamic load fixtures for applying a controllable torsion load capable of replicating acceptance test procedures and nonlinear load conditions.
2. Description of the Related Art
The use of rotary mechanical systems to power automobiles, drive robotics, actuate flight control systems on airplanes and missiles and many other mechanical systems is ubiquitous throughout our economy. The use of a motor to rotate a shaft to actuate these various systems is a cost effective and reliable way to convert electrical energy into a mechanical force. In many applications such as found in an automobile, the motor rotates the drive shaft at a high and relatively constant rate. Therefore, the shaft has a large range of motion but a relatively small frequency content. In other applications such as found in an airplane, the motor rotates the drive shaft over a small range of motion, less than ten turns or even a single turn, to actuate flight control. In applications such as found in a missile, the motor rotates the drive shaft over a small range of motion but at a very high rate to control the deployment of the fins, canards or wings to guide the missile.
The different applications and environments produce a wide range of load conditions for the rotary mechanical systems. Before a design can be approved or, in some cases, a particular system fielded, it must be tested to determine how the system performs under certain load conditions. More specifically, when a torque is applied to the shaft how does the system respond?
Conventional techniques for testing control actuation systems (CAS) used for steering control of guided missiles and flight vehicles typically employ fixed end torsion bars to simulate aerodyanic loads encountered in flight. As shown in FIG. 1, a CAS 10, referred to as a unit under test (UUT) during testing, includes an actuator device 12 and a drive shaft 14 that rotates about an axis 16. A conventional test fixture 20 includes an interface bracket 22 that is bolted to the end of drive shaft 14, a torsion bar 24 that is rigidly mounted on the interface bracket along axis 16, and a plate 26 that fixes the other end of the torsion bar to a mechanical ground. A measuring device 28 is placed around the interface bracket to measure the angular rotation of the torsion bar when the unit under test is activated. The amount of torque in the torsion bar, hence the load applied to the UUT is proportional to the angle of rotation.
This approach limits the evaluation to quasi-static conditions at small deflection changes and prohibits testing of the CAS under desired acceptance test procedures and realistic load environments demanded of typical flight scenarios. Specifically, a “torque at rate” test procedure requires the application of a constant torque load for a constant rotation rate of the drive shaft. Typical flight scenarios produce rapidly changing nonlinear load conditions. Clearly a fixed end torsion bar cannot replicate these conditions. Furthermore, to test the UUT over a range of load conditions albeit quasi-static an operator must replace the torsion bar with a different torsion bar having different stiffness properties. This is very inconvenient and slow.